1. Field of the Invention
This invention relates generally to a technique for reducing FM intrusion in an AM receiver and, more particularly, to a circuit design technique for reducing the intermodulation products produced by the FM front end circuity of an AM/FM receiver that effect AM reception when the receiver is in the AM mode by switching an FM tuned tank circuit to a reference potential.
2. Discussion of the Related Art
As is well understood in the art, radio receivers are responsive to radio frequency (RF) signals broadcast from a transmission antenna to convert the signals into a desirable format, such as speech or music. An antenna associated with the receiver captures the electromagnetic energy of the RF signals from the surroundings, and converts this energy into electrical currents that are subsequently processed. Typically, a radio receiver will be separated into an amplitude modulation (AM) portion and a frequency modulation (FM) portion. In the United States, the AM portion is tunable to RF signals in the frequency band from 530 to 1710 kHz, and the FM portion is tunable to RF signals in the frequency band from 88.1 to 107.9 MHz.
There are many things that affect reception capabilities of the received RF signal, resulting in a poor signal-to-noise ratio. The RF signal can become very weak, or attenuated, and not be able to overcome noise sources in the receiver. This occurs because of many reasons, such as low power transmission, distance of the receiver from the transmitter, obstacles between the transmitter and the receiver, etc. Further, there can be one or more high-power, adjacent, alternate or co-channel signals present which causes interference and degrades the desired signal quality.
Strong RF signals can provide problems as well. The electrical circuitry in the front end of a radio receiver provides selectivity, amplification and frequency translation. This circuitry includes active components which are only linear over limited voltage ranges. The presence of strong RF signals can force these components into their non-linear operating ranges. The result can be intermodulation products which are additional frequency components that were not present in the RF signals, and were generated as a result of the non-linearity in the system circuitry.
FM intrusion is a problem that may exist in an AM receiver as a result of the reception of strong FM signals. FM intrusion is an intermodulation product condition that results from two FM frequencies that are spaced apart by a band width that is in the AM frequency band. In the United States, FM transmissions are regulated to be spaced 200 kHz apart. This difference in adjacent FM transmissions results in frequencies that could fall in the AM frequency band range at 600, 800, 1000, 1200, 1400 and 1600 kHz. If two FM broadcast sources are positioned geographically close together, are sufficiently strong and are a derivative of 200 kHz apart, FM intrusion may become a problem. In other parts of the world FM stations are spaced 100 kHz apart which results in more opportunity for FM intrusion within the AM band.
Many prior art techniques have been attempted to improve FM intrusion in the AM portion of the receivers. These methods generally either try to improve the linear range of the front end circuitry of the AM receiver, or use filtering techniques to keep the 200 kHz frequency components from getting into the AM receiver section of an AM/FM receiver. One particular technique uses an isolation choke positioned in series with the input to the AM section of the receiver. The isolation choke is a high impedance device to the high frequency components of the FM signal, and a low impedance device to the lower frequency AM signal. Therefore, the FM signals are prevented from reaching the AM tuner in the AM portion of the receiver.
The above described isolation choke has been relatively successful in preventing FM signals from reaching the AM tuner directly from the receiver antenna. However, the FM signals have been able to reach the AM tuner along certain "sneak paths," i.e., unobvious connections in the circuitry. One particular sneak path is through the ground plane of the receiver. This allows the FM signals to reach the AM tuner circuitry, and thus provide a source of FM intrusion.
What is needed is a technique for further reducing intermodulation products and FM intrusion in a combination AM/FM receiver. It is therefore an object of the present invention to provide such a technique.